Viruses are extremely small, much smaller even than bacteria (see Figure 15.2). The light microscope is unable to focus in on them, but they appear as rods or spheres when seen under an electron microscope. The virus particle is composed of a DNA or RNA core surrounded by a protective protein coat. On entering a plant cell, the virus takes over the organization of the cell nucleus in order to produce many more virus particles. Since the virus itself lacks any cytoplasm cell contents, it is often considered to be a non-living unit. Some details of its classification are given in Chapter 4.
The virus's close association with the plant cell nucleus presents difficulties in the production of a curative virus control chemical that does not also kill the plant. No established commercial 'viricide' has yet been produced against plant viruses.
In recent years the broad area called 'virus diseases' has been closely investigated. Virus particles have, in most cases, been isolated as the cause of disease, e.g. cucumber mosaic. Other agents of disease to be discovered are viroids (e.g. in chrysanthemum stunt disease) and these are smaller than viruses. Mycoplasmas (the cause of diseases such as aster yellows) are a group of bacteria that induce symptoms similar to those produced by viruses.
Spread. A number of organisms (vectors) spread viruses from plant to plant and then transmit the viruses into the plant. Peach-potato aphid is capable of transmitting over 200 types of virus (e.g. cucumber mosaic) to different plant species. The aphid stylet injects salivary juices containing virus into the parenchyma and phloem tissues, enabling the virus to then travel to other parts of the plant. 'Persistent virus transmission' is seen in some vector/virus combinations such as peach-potato aphid/potato virus X, and Xiphinema dagger nematode/arabis mosaic where the virus is able to survive and increase within the vector's body for several weeks. In many vector/virus combinations such as plum pox, the virus survives only briefly as a contaminant on the insect's stylet. Other vector/virus combinations include bean weevils/broad bean stain virus; and Olpidium soil fungus/big vein agent on lettuce.
Other important methods of spread involve vegetative material (e.g. chrysanthemum stunt viroid and plum pox), infected seed (e.g. bean common mosaic virus), seed testa (e.g. tomato mosaic virus) and mechanical transmission by hand (e.g. tomato mosaic virus).
Symptoms. The presence of a damaging virus in a plant is recognizable to horticulturists only by means of its symptoms. For confirmation, they may need to consult a virologist, whose identification techniques include electron microscopy, transmission tests on sensitive plants such as Chenopodium species, and serological reactions using specific antiserum samples.
Leaf mosaic, a yellow mottling, is the most common symptom (e.g. cucumber mosaic virus). Other symptoms include leaf distortion into feathery shapes (cucumber mosaic virus), flower colour streaks (e.g. tulip break virus), fruit blemishing (tomato mosaic and plum pox), internal discolouration of tubers (tobacco rattle virus causing ' spraing ' in potatoes) and stunting of plants (chrysanthemum stunt viroid).
Symptoms similar to those described above may be caused by misused herbicide sprays, genetic 'sports', poor soil fertility and structure (see deficiency symptoms) and mite damage.
In the following descriptions of major viruses, Latin names of genus and species are not included, since no consistent classification is yet accepted.
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